By far the most annoying trophy to get in this game, especially now when it's old and there are far less players playing it online now. You better hurry up too because the servers will probably get shut down in a few years since they recently shut down Skate 2's servers.
What you need to do is to make a team, or join a team. The team must consist of minimum 6 players in order to get the trophy.
Tips on getting teammates is obviously first ask all your friends irl if they can help you get it or if they want to get it too if they don't have it already. This helps tremendously, I had 2 friends who wanted to do it so we only had to find 3 more.
You can also look on the Skate 3 forums and see there if people wants to do it but note, there is far less people on there now too since the game is old so my biggest recommendation after asking irl friends is go to Super Ultra Mega Park freeskate, hook up your keyboard to the PS3 and ask everyone you see if they want to get the trophy. This is extremely annoying but if you do it like this you can get it in just a few hours. (I've gotten it twice, it took 4 hours the first time and 2 hours the second time.) Beware of children. It has happened to me multiple times that when we finally got 6 players and we invited everyone that some logged off, some switched games and didn't want to change back and some have NO patience at all and leave if they don't get the trophy within a minute.
So lastly, when you've finally got your 6 man team you want to go to Online Challenge Map, go to Freeskate and choose your location and then pick team freeskate. As fast as you're in, invite all your team players via the contacts in the menu.
Hopefully this helps someone if they're still trying to get it!